1. Field of Invention
The present invention generally relates to nickel-gold plating processes for semiconductor and insulating substrates, and, more particularly, to such a process characterized by significantly enhanced bond strength between the gold and the nickel.
2. Description of the Prior Art
In the metallization of ceramic and other substrates for the purpose of providing electrical contact terminals, nickel is often deposited as a base layer. Nickel, however, is known to oxidize rather readily and the resulting oxide interferes with the establishment of sound electrical contact. Accordingly, steps have been taken in the prior art to inhibit or prevent the oxidation of nickel by the application of a material, usually gold, over the nickel as soon as the nickel layer has been formed. In some instances where a thick layer of gold is desired, such as where thermocompression bonding is required, a thick layer of gold is electroplated on the nickel. This is disclosed in the IBM.RTM. Technical Disclosure Bulletin "Process For Preventing Chip Pad Corrosion" by M. M. Haddad et al, Vol. 19, No. 12, May 1977, page 4581. A similar thick gold electroplating process is described in the Technical Disclosure Bulletin "Selectively Electroplating Pad Terminals on an MLC Substrate" by M. M. Haddad, Vol. 20, No. 9, February 1978, page 3443.
In the Technical Disclosure Bulletin "Nickel/Gold Diffusion Barrier" by J. R. Lynch, Vol. 14, No. 4, September 1971, page 1099, a diffusion barrier is formed between a layer of electroplated gold and an adjacent layer of nickel in order to prevent the diffusion of the nickel into a silicon device which is to be bonded to the gold layer. The diffusion barrier is formed by a layer of electroplated gold placed on the nickel and then fired in a hydrogen atmosphere at about 700.degree. C. The barrier layer is left in place and later covered by an additional layer of gold to which the silicon device is bonded.
It has been found that the process of the above-cited 1971 Technical Disclosure Bulletin also is suitable for use in applications where thermocompression bonding is desired and where a diffusion barrier, as such, is not needed. More recently, however, it has been noted that the "pull strength" of a thermocompression bond established upon a metallurgical combination of a second gold layer upon a first gold layer (diffusion barrier), upon a nickel layer, upon a substrate, is insufficient. In some cases, it is necessary that the pull strength be increased by a factor of 50% or more.